Introduction
Lung cancer has the highest mortality rate among cancer-related diseases based on the latest worldwide report [
1]. Non-small cell lung cancer (NSCLC) accounts for approximately 85% of cases among 1.8 million newly diagnosed patients [
1,
2]. Lung adenocarcinoma (LUAD) and squamous cell carcinoma (LUSC) are two main subtypes. Despite advances in the diagnosis and treatment of NSCLC, the five-year survival rate is still poor [
3]. Therefore, a deeper understanding of the pathogenesis of lung cancer is urgently needed.
AKT (protein kinase B, PKB) family members are well known for their roles in regulating tumorigenesis and development [
4]. AKT1, AKT2 and AKT3 belong to the AKT family. All the AKT family members contain similar protein structures, such as the N-terminal, PH domain, catalytic domain and C-terminal regulatory domain, although the members are encoded by different genes [
5]. Tissue distributions are varied among these family members. AKT1 and AKT2 are widely expressed in human tissues, while AKT3 is mainly distributed in brain tissue [
4]. Among these three, AKT2 is much more closely associated with cancer cell metabolism, proliferation, cell survival, metastasis, angiogenesis and drug resistance [
6]. In breast cancer, inhibition of AKT2 can not only effectively prevent the transformation of mesenchymal non-cancer stem cells (non-CSC) into epithelial cells but also reduce the invasive and colony formation abilities of non-CSC and CSC [
7]. In colon cancer, the expression of AKT2 can affect the DNA repair ability and radiosensitivity [
8]. In NSCLC, AKT2 can affect tumor cell survival and chemotherapy sensitivity [
9]. Thus, AKT2 is considered a promising target for cancer-targeted therapy.
MicroRNAs are single strand small noncoding RNAs that partially or completely bind to the 3′-UTR region of target mRNAs to modulate gene expression, resulting in the regulation of proliferation, differentiation, apoptosis and metastasis in cancer cell progression [
10,
11]. MiR-124 expression has been reported to be widely downregulated in many cancers, such as breast cancer, colon cancer, glioma, lymphoma, NSCLC and so on [
12‐
16]. Accumulating reports have demonstrated the roles of miR-124 in the occurrence and development of a variety of tumors. In neuroblastoma, miR-124 induces neuroblastoma differentiation by downregulating the expression of the transcription factor ELF4 [
17]. In bladder cancer, miR-124 negatively modulates EDNRB to suppress proliferation and induce apoptosis in tumor cells [
18]. Although miR-124 expression was suggested to be downregulated in NSCLC, the molecular mechanism underlying the involvement of the miR-124/AKT2 axis in mediating cell proliferation and metastasis is not fully understood, especially in LUAD subtypes [
16,
19,
20].
In our current study, we proved that AKT2 expression was upregulated in NSCLC tissues. Knockdown of AKT2 expression could attenuate cell proliferation via cell cycle arrest. Moreover, migration was also inhibited by reversing the EMT process and regulating the MMP family. In addition, we confirmed that miR-124 could directly target the 3′-UTR of AKT2 and thus inhibit AKT2 expression in vivo and in vitro. Taken together, our data identify a novel mechanism by which the miR-124/AKT2 axis mediates carcinogenesis and provide new potential therapeutics for LUAD.
Materials and methods
Patients and tissue samples
Tissue samples from 45 NSCLC patients and matched noncancerous tissue samples collected between 2012 and 2016 were obtained from the First Affiliated Hospital of Soochow University. All cases were confirmed by experienced clinicians and pathologists, and no patient received any relevant treatment before sampling. All collected samples were stored at − 80 °C. Each patient involved in the research signed a written informed consent form, and this study was approved by the Ethics Committee and Institutional Review Board of the First Affiliated Hospital of Soochow University. All the methods used in the study are based on the approved guidelines.
Immunohistochemisty (IHC) staining
Tissue samples were fixed with 4% paraformaldehyde and embedded in paraffin. After baking, deparaffinization and rehydration, slides were immersed in 3% H2O2 for 15 min, heated at 95 °C for 25 min and cooled at room temperature. These sections were then incubated with an anti-AKT2 polyclonal antibody (diluted 1:200, Proteintech, 17,609–1-AP) and an anti-KI67 antibody (diluted 1:400, Cell Signaling Technology, 9449) at 4 °C overnight and visualized using diaminobenzidine as the chromogenic substrate (DAB kit, ZSGB-Biotechnology Co., Ltd., Beijing, China). Finally, the sections were counterstained with hematoxylin and examined by optical microscopy. The staining results for AKT2 were evaluated based on the staining intensity and the percentage of positively stained cells.
Cell culture
Cells were provided by the Cell Bank of the Chinese Academy of Sciences (Shanghai, China). RPMI 1640 medium supplemented with 10% fetal bovine serum (Gibco, Carlsbad, CA, USA), penicillin (100 U/ml) and streptomycin (100 ng/ml) was used for cell culture. All cell lines were maintained in 37 °C humidified incubators with 5% CO2.
Western blot analysis and antibodies
Western blot analysis was performed as we described previously [
21]. In this study, the antibodies used were anti-AKT2 (17609–1-AP, Proteintech, China); anti-pAKT (Ser473) (D9E), anti-AKT, anti-pErk (Thr202/Tyr202) (D13.14.4E), anti-Erk (137F5), anti-Slug (C19G7), anti-MMP9 (603H), anti-MMP7 (D4H5), and anti-MMP2 (D8N9Y) (Cell Signaling Technology, Danvers, MA, USA); anti-N-cadherin and anti-vimentin (RV202) (BD Biosciences, USA); and anti-β-actin and anti-mouse or anti-rabbit secondary antibodies (Cell Signaling Technology). Each experiment was performed in triplicate.
RNA isolation and quantitative real-time PCR analysis
The detailed processes were performed as we previously described [
22]. The primer sequences used in this study are listed in Supplementary Additional file
1 Table S1 (Sangon Biotech, Shanghai, China). MiR-124 and U6 primers were purchased from RiboBio Co. (Guangzhou, China). The CT values of
AKT2 mRNA and miR-124 were normalized to those of
ACTB mRNA and U6, respectively. The
△△Ct method was applied to calculate the relative quantities of these mRNAs. Each experiment was performed in triplicate.
MicroRNA (miRNA) and small interfering RNA (siRNA) transfection
A549 and H1299 cells were seeded in the 6-well plates. When the cell intensity reached 60%, we replaced the medium with 1.5 ml fresh serum-free RPMI-1640 medium for 2 h and then 250 μl serum-free medium containing 5 μl Lipofectamine 2000 transfection reagents or 5 μl siRNA /microRNA (50 nmol per well) were mixed for 15 min at room temperature. Later, the above mixture was added into 6-well plates. Six hours later, the cell supernatant was changed with 2 ml medium with 10%FBS. After 48–72 h transfection, the cells were collected for further experiments. MiR-124 mimic and negative control were purchased from GenePharma (Shanghai, China). The siRNAs specific for AKT2 were provided by Shanghai GenePharma Company. The target sequences of the siRNAs were as follows: siRNA-AKT2–1: 5′-GCUCCUUCUAUUGGGUACAATT-3′, siRNA-AKT2–2: 5′-GCGGAAGGAAGUCAUCAUUTT-3′, and siRNA-AKT2–3: 5′-GGUUCUUCCUCAGCAUCAATT-3′.
Cell proliferation was evaluated using Cell Counting Kit-8 (Beyotime Institute of Biotechnology). Briefly, cells were cultured in 96-well plates and seeded at 3000 cells per well. At 24 h, 48 h and 72 h, 10 μl CCK-8 dye was added to each well and incubated at 37 °C for 3–4 h. Then, the absorbance was measured at 450 nm and 630 nm using a spectrophotometer (Thermo Fisher Scientific). For the colony formation assay, cells were cultured for 7–10 days until foci formed. The cells were then fixed with methanol and stained with crystal violet. For the EdU assay, we performed the experiment according to the kit instructions. A cell-light EdU Apolo567 in vitro kit was purchased from RiboBio Co. The experiments were performed in triplicate.
Cell migration and invasion assays
For the migration assay, 3 × 104 transfected cells suspended in medium containing 1% FBS were added to the upper chamber, and 800 μl normal medium containing 10% FBS was added to the lower chamber. For the invasion assay, the upper chamber was coated with a Matrigel matrix at a 1:6 dilution (BD Science, Sparks, MD, USA). Then, 1% FBS medium containing 5 × 104 tumor cells was added to the upper chamber, and 800 μl 10% FBS medium was added to the lower chamber. After culturing for 24 h, the cells were fixed with methanol, stained with crystal violet, and imaged under a microscope. The transwell chambers used in our study were purchased from BD Biosciences. Each experiment was performed independently in triplicate.
Flow cytometry analysis
For cell cycle assay, transfected tumor cells were washed with PBS, suspended in 70% ethanol and then fixed at 4 °C overnight. Then, the cells were stained with a mixed propidium iodide solution and incubated at 37 °C for 30 min. For apoptosis assay, transfected tumor cells were stained with an Annexin V/PI kit (Beyotime, Shanghai, China). Finally, the cells in both experiments were evaluated using a fluorescence-activated cell sorting (FACS) Caliber system (Beckman Coulter, Brea, CA, USA). Each experiment was performed independently in triplicate.
Dual-luciferase reporter assay
143-bp sequence of the 3′-UTR of AKT2 containing the putative miR-124 binding site or mutated target site were synthesized and cloned into the pGL3 basic vector (Promega, Madison, WI, USA) which were named WT (wild-type AKT2 3′-UTR fragment) and Mut (mutant AKT2 3′-UTR fragment). A549 and H1299 cells were seeded in 24-well plates and co-transfected with WT or Mut plasmids along with either miR-NC or miR-124 mimic using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA). At the same time, pRL-TK Renilla Luciferase Reporter Vector (Promega) was transferred into each well. After incubated for 48 h, the cell lysates were harvested. The luciferase activity was assessed by the Dual-Luciferase Reporter Assay Kit (Promega) and then standardized with renilla luciferase activity.. Each experiment was performed independently in triplicate.
Tumorigenesis in nude mice
Female BALB/c nude mice were obtained and bred in the Experimental Animal Center of Soochow University. In total, 2 × 106 A549 cells were injected subcutaneously into the mice. Fifteen days after inoculation, a miR-124 agomir (RiboBio Co.) and miR-NC agomir were used for intratumoral treatment at a dose of 2 nmol per tumor. Intratumoral treatment was performed three times a week for a total of seven times. Xenograft tumor volume and body weight of mice were measured every 2–4 days for a total of 36 days. We used the volume measurements to evaluate the growth of tumors, and tumor volume was calculated by measuring the length and width of a xenograft tumor and using the following formula: (Volume = length × width2)/2.
Statistical analysis
Student’s t-test was used for statistical analysis, and P < 0.05 was considered significant. All statistical analyses were performed using GraphPad Prism 5.0 (GraphPad, San Diego, CA, USA) and SPSS 7.0 software (SPSS, Chicago, IL, USA). All results are presented as the mean ± SD (standard deviation). Kaplan-Meier methods were used for survival analysis, and survival curves were compared by the log-rank test.
Discussion
AKT2, characterized as a serine/threonine protein kinase, has been proven to facilitate and contribute to tumor development in addition to its roles in angiogenesis, myoblast differentiation, glucose metabolism and inflammatory disease regulation [
24]. AKT2 has been discovered to be overexpressed in human cancer tissues, including lung cancer, breast cancer, and colon cancer tissues [
25‐
27]. An increasing amount of evidence supports that AKT2 can be regarded as a prognostic indicator in cancer patients. In osteosarcoma, AKT2 expression is significantly higher in cancerous tissues than in noncancerous tissues, implying shortened event-free survival and overall survival [
28]. In meningiomas, AKT2 expression is negatively correlated with patient recurrence-free survival [
29]. In pancreatic cancer, elevated AKT2 expression indicates shortened progression-free survival and overall survival [
30]. In our present study, we confirmed that AKT2 is highly expressed in LUAD and closely related to the survival of LUAD patients. Although AKT2 is also highly expressed in lung squamous cell carcinoma, our study found that it was not related to the prognosis of lung squamous cell carcinoma, partially owing to the small sample size of evaluated patients. On the other hand, there might be other factors that contributed to the differences between the LUAD and LUSC subgroups, but this point needs further study.
As an important downstream component of the PI3K pathway, activated AKT2 can promote the transcription of the downstream substrate rapamycin (mTOR) and transcription factors Forkhead family (FOXO), which is involved in protein synthesis, cell proliferation, metastasis and survival processes [
31]. Our study showed that A549 and H1299 cells contained the highest AKT2 expression levels among four adenocarcinoma cell lines tested. Therefore, we chose A549 and H1299 cells to explore the role of AKT2 in LUAD. We silenced AKT2 expression with specific siRNAs in these two cell lines. The results showed that silencing AKT2 expression decreased cell proliferation via cell cycle arrest, while apoptosis was not affected. Moreover, migratory and invasive abilities were inhibited in the AKT2-knockdown group.
Although the role of AKT2 in human cancer has been well established, the specific mechanism of AKT2-mediated tumor development remains to be explored. Studies have demonstrated that activated AKT2 can result in transcriptional control, such as the suppression of p21, Bax, Bad and procaspase-9 expression or the promotion of insulin-like growth factor receptor-1 expression [
9,
32‐
35]. Our data showed that the mRNA and protein expression levels of EMT markers (N-cadherin, Vimentin and Slug), MMP7, p-AKT, and p-Erk were consistently decreased after AKT2 knockdown in LUAD cells. Therefore, the results of our current research illustrate the role of AKT2 as a tumor oncogene, promoting cell proliferation and metastasis in LUAD.
Previous studies have proven that AKT2 can be mainly regulated by upstream PI3K and PTEN. In contrast to the stimulatory role of PI3K, PTEN negatively regulates the AKT2 signaling pathway [
35]. Apart from this signaling pathway, AKT2 can also be regulated by microRNAs [
36]. MicroRNA/mRNA interactions are regarded as a fundamental and epigenetic gene-regulatory mechanism. Based on the analysis of four bioinformatic databases including miRDB, miRWalk, miRTarBase and TargetScan, seven microRNAs were predicted to potentially bind to AKT2. MiR-124 was found to potentially contribute to AKT2 overexpression. MiR-124 has been reported to play critical roles in tumor formation and development in multiple cancers, such as lung, breast and liver cancer [
16,
37,
38]. In liver cancer, miR-124 overexpression reduces the expression of CLIC1, thereby reducing the migration and invasion of liver cancer cells [
38]. In lung adenocarcinoma, miR-124 regulates cell proliferation, migration, and invasion by directly targeting SOX9 [
23]. However, the potential association between miR-124 and AKT2 in LUAD is unclear. In the current study, we first proved that miR-124 can bind to the wild-type 3′-UTR but not a mutant 3′-UTR via a luciferase assay. Additionally, we found that the expression of miR-124 in NSCLC tissues was lower than that in normal lung tissues and negatively correlated with AKT2 expression. Then, we overexpressed miR-124 in A549 and H1299 cells to evaluate its effects on proliferation, metastasis and invasion. Our results showed that AKT2 expression could be significantly decreased by miR-124 overexpression. Consistently, CCK-8 and transwell assays confirmed that miR-124 overexpression could significantly reduce cell proliferation, metastasis and invasion. Furthermore, we found that upregulated miR-124 expression could inhibit the expression of EMT markers, MMPs, p-AKT, and p-Erk. Finally, our experimental results were verified in vivo
.
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